The ability to recognize individual faces is an essential skill for many animals. Humans do it well, as do dogs, sheep, chimpanzees and crows, among others. These are all highly social species, which is no coincidence. Individual recognition plays a key role in many interactions within social groups, where the ability to tell friends from foes is so essential.
As sociality is strongly linked with facial recognition, we expect that highly social species would be able to tell individuals apart by looking at their faces. However, when a graduate student at Cornell discovered in 2001 that a species of highly social wasps is adept at facial recognition, it was big news.
Scientist Elizabeth Tibbetts noticed variation in the yellow markings on the paper wasp Polistes fuscatus, and hypothesized that this variability allowed individuals to tell each other apart. To test this idea, she changed the wasps’ facial markings with paint and observed that these altered wasps received more aggression from other wasps in the nest than wasps who were painted in such a way that it did not alter their markings. The aggression towards wasps whose identity had been obscured declined over time, suggesting that their nestmates became familiar with their new look.
Years later, another study with these wasps showed that they are especially adept at learning individual faces, rather than having a generalized skill for pattern recognition. Wasps were trained to navigate a T-shaped maze. At the decision point, there were images of two different wasps, and the wasps learned that a reward was available if they followed the path marked by one of those wasps but that the other wasp led them to a place with no reward. Wasps learned the task quickly and well when the cues indicating the right and wrong paths were wasp faces. When the cues were simple shapes or other images, the wasps learned much slower and less effectively.
Another social species, the tropical hover wasp, uses facial recognition to defend their nests. These wasps live in colonies of fewer than 10 individuals, but as many as 150 nests can be aggregated very close together. Colonies face regular visits by intruders from nearby nests and act aggressively towards them. In their experiment, scientists dangled captured and killed wasps that had received different treatments near nests and observed the response of the wasps on the nest to that “intruder”. There were four types of intruders: a nestmate with the right colony odor, a nestmate with the odor from another nest, an unknown wasp with the nestmate odor and an unknown wasp with the odor from another nest. Researchers showed that wasps rely on facial recognition more than on scent to determine if a wasp belongs there, biting individuals with unknown faces far more than those with familiar faces, regardless of the odor. That was a surprise because the use of colony odor to recognize nestmates has been well documented in many wasp species.
Facial recognition—yet another reason for people to feel a kinship with other animals.